It'd be great if I could view the temperature in both C and F. I prefer C and my wife prefers F, so when I know how cold/hot it'll feel she doesn't and vice versa.
I do not think most users will need this, and it would be a lot of work to implement it, having to care about lots of additional cases for presentation. Further there would be many issues with the plugin taking too much space etc. As a different solution, how about adding two instances of the plugin to the panel? You can configure one to show the temperature in Fahrenheit, the other to show it in Celsius. Later versions will allow you to hide the scrollbox and show those values in the tooltip in order to save valuable panel space.